The present invention relates generally to signal integrity and, more specifically, to reducing the effects of electrical impedances.
To terminate a high-speed electrical bus, dissipative components such as resistors having impedance matched to the impedance of the bus"" transmission line are commonly used in series at the end of the bus. The matched impedance absorbs and thus reduces or eliminates signal reflections that bounce back on the bus to the signal source. However, if the end of the bus is connected to an active component, such as a processor, the active component itself can serve as a termination. When the active component is redundant, e.g., for improved performance or reliability, it is desirable to make that component optional.
To replace an optional component when it is removed from the system, one implementation uses a removable printed-circuit assembly (PCA) consisting of a printed-circuit board (PCB) upon which high-precision resistors are placed. PCBs usually include vias, which are conducting material in tunnel forms from one side of the PCB to the other side. On thin PCBs, vias are capacitive. However, on thick PCBs, vias behave as added electrical inductance, which, together with parasitic inductance from resistors used on the PCBs, can greatly affect circuits"" performance.
When a signal flows through the vias and the resistors, the signal carries with it a current flow seeking to return to the original source driver that drives the signal. The current flowing through big loops of vias with high inductance can cause signal spikes and electrical crosstalk. Multiple signals intermingled with the returned current can also cause crosstalk.
A transmission line or an electronic component placed near a ground source (e.g., a ground line, a ground plane, etc.) is said to be referenced against ground. Similarly, the same transmission line or electronic component placed near a power source is said to be referenced against power. It is preferable that a signal flows through a path having components referenced against only one type of source, i.e., either power or ground, but not both. If a signal path having components referenced against both ground and power, then the high-frequency returned current experiences power-ground discontinuity, which can cause the xe2x80x9cbig loopxe2x80x9d inductance effects discussed above because the current has to flow from power to ground in a longer path.
Based on the foregoing, it is desirable that mechanisms be provided to solve the above deficiencies and related problems.
The present invention, in various embodiments, provides techniques for reducing effects of electrical impedance. In one embodiment, the impedance is in the form of inductance and arises from vias in a termination PCB and from resistors used on the PCB. In one embodiment, a power plane is placed near the resistors. Additional power and ground planes are created in parallel among themselves and perpendicular to the vias, which cause capacitance to be formed between each pair of the ground and power planes, the ground planes and the vias, and the power planes and the vias.
In one aspect, the power plane near the resistors and the formed capacitance allow the high-frequency returned currents to flow through a smaller loop and thus be affected by a smaller inductance. Additionally, the created capacitance reduces both the total impedance of the vias and the resistors and any impedance that result from power-ground discontinuity.